Mid-Paleozoic dextral transpression in a northern portion of the Appalachian orogen

The formation of mountain belts through the collision of continents, microcontinents, and island arcs is the fundamental way in which continents grow. Transpression, tectonism involving components of strain lateral and orthogonal to the orogen, is common because plate convergence vectors are rarely exactly orthogonal to plate boundaries or continental margins. Therefore, an understanding of the thermal and deformational processes that occurred within an ancient transpressive orogen provides insight into the evolution of numerous orogens through time and around the globe.; Previous studies have suggested that eastern central Maine experienced dextral transpressive orogenesis during the Devonian, including a broad, ductile phase of dextral non-coaxial deformation. However, the spatial and temporal extent, and significance of this deformation was unclear. In this dissertation, the existence of a NE-striking non-coaxial S3 foliation distinct from the NE-striking axial-surface-parallel S2 foliation is established. S3 foliation decreases in intensity away from the dextral strike-slip Norumbega fault system to the northwest, forming a fabric intensity gradient approximately 60 kilometers wide, comparable to fabric gradients associated with other major strike-slip faults. Analysis of shortened, extended, and shortened-then-extended fibrous calcite veins in limestone is used to estimate the vorticity of flow, a quantification of the amounts of pure shear vs. simple shear, for the D3 non-coaxial deformation. The resulting kinematic vorticity number of 0.67 confirms simple-shear-dominant non-coaxial strain on the margin of the deforming area during S3 development. Microstructural analysis of the geometries and orientations of inclusion trails preserved in porphyroblasts suggests that porphyroblasts rotated with respect to foliation and geographic coordinates. Rotation of early metamorphic phases requires that the non-coaxial deformation began as early as the Late Silurian. Petrographic analysis suggests that metamorphism consisted of a single, regional heating event that progressed from east to west over approximately 50 m. y., followed by distributed Late Devonian plutons. In-situ microprobe analysis of monazite included in porphyroblasts and in matrix phases produced Silurian-Devonian ages that are consistent with published isotopic ages for the area. These data, combined with the structural and microstructural analyses above, confirm that dextral-transpressive tectonism was significant, and active in eastern central Maine by the Late Silurian.